Dmitry Kisler performs image classification using Keras in both Python and R:
From the plots above, one can see that:
the accuracy of your model doesn’t depend on the language you use to build and train it (the plot shows only train accuracy, but the model doesn’t have high variance and the bias accuracy is around 99% as well).
even though 10 measurements may be not convincing, but Python would reduce (by up to 15%) the time required to train your CNN model. This is somewhat expected because R uses Python under the hood when executes Keras functions.
This is just one example, but the results are about what I’d expect.